1. Field of the Invention
The present invention relates to nuclear magnetic resonance tomography devices, and in particular to such a device for visual presentation of interior regions of an examination subject having coils for generating fundamental and gradient magnetic field in which the examination subject is disposed, and a surface coil for acquiring a signal corresponding to the displacement of the nuclear spin of the examination subject from an equilibrium position.
2. Description of the Prior Art
A nuclear magnetic resonance tomography apparatus having coils for generating a fundamental magnetic field and coils for generating a gradient magnetic field is described in U.S. Pat. No. 4,520,315. This apparatus does not make use of a surface coil. In this device, it is possible to deflect the nuclear spins of an examination subject from a preferred orientation, which is determined by a fundamental magnetic field. The deflection occurs by means of a high-frequency excitation pulse. A signal is generated when the nuclear spin subsides after the excitation pulse, the frequency of this signal depending upon the strength of the fundamental magnetic field. By superimposing a field gradient on the homogeneous fundamental magnetic field, so that the magnetic field distribution is spatially varied, it is possible to locate areas responding differently to the pulse by measuring the respective frequencies in the region of interest. A cross-section image of the examination subject can be generated in this manner by changing the direction of the field gradient. Excitation in a layer of the examination subject is undertaken by acting on the fundamental magnetic field with a further field gradient so that excitation of the nuclear spin occurs only in the selected layer. This is possible because the excitation occurs only with a frequency which is specifically allocated to the magnetic field in the selected layer.
It is known to improve the chronological resolution in the region of interest in such nuclear magnetic resonance tomography devices by employing a surface coil which has an improved signal-to-noise ratio. Transmission from the surface coil to processing circuitry can then be undertaken with a larger antenna, called a body resonator, for generating a uniform field in the region of the surface coil. The signals generated by the nuclei are received with the surface coil, however, it is also theoretically possible to transmit as well as receive with such a surface coil.
When such a conventional surface coil is placed on the body region to be examined, a frequency detuning caused by electrical losses in the body tissue occurs.